Public Interest Transportation Forum - http://www.bettertransport.info/pitf
The Supplemental Draft Environmental Impact Statement Chapter 9 on tolling impact for the Alaskan Way Viaduct replacement with a tunnel provides the results of Washington State DOT modeling of traffic diversion. The diversion is from SR 99 in its present elevated viaduct configuration to City of Seattle surface streets in the 2015 proposed scenario of a tolled four lane highway tunnel connecting Aurora Avenue near Seattle Center to a south portal near the stadiums.
The following is a map on page 213 of the SDEIS with annotations made by PITF from the text description of the tolls represented by the three tolling scenarios A, C, and E described earlier in the same chapter. The four maps collectively indicate that the traffic volume of 117,000 cars per day on the Alaskan Way Viaduct would be partially diverted to Seattle city streets depending on how high the tolls were set for the tunnel. With the range of tolls shown, the diversion would be between 39,600 and 70,300 per day.
Even if there were no tolls at all, the diversion would be 30,400 per day, because the ramps into and out of the tunnel are different than the entrance and exit ramps for the present Viaduct. This point was made in a Seattle Times story on October 10, 2010 by Mike Lindblom.